Metallic hydroxides, e.g. nickel hydroxide, cadmium hydroxide, and silver hydroxide, are widely used as electrodes for electrochemical cells or batteries. These electrodes generally influence cell performance and lifetime more than other cell components. At the same time, their manufacture is often the most difficult in terms of process control, complexity, production time and reproducibility. A variety of manufacturing methods exist. See, e.g. Falk & Salkind, Alkaline Storage Batteries (Wiley 1969) pp. 125-128; Kandler, U.S. Pat. No. 3,214,355, issued Oct. 26, 1965; Beauchamp, U.S. Pat. No. 3,573,101, issued Mar. 30, 1971; Beauchamp, U.S. Pat. No. 3,653,967, issued April 4, 1972; Beauchamp, et al. U.S. Pat. No. 4,032,697, issued June 28, 1977; Beauchamp, U.S. Pat. No. 4, 228,228, issued Oct. 14, 1980; Crespy et al, U.S. Pat. No. 4,132,606, issued Jan. 2, 1979; Gutridge, U.S. Pat. No. 3,997,364, issued Dec. 14, 1976; Oswin U.S. Pat. No. 4,224,392; Gutjahr et al., U.S. Pat. No. 3,926,671, issued Dec. 16, 1975; Gutjahr et al., U.S. Pat. No. 4,273, 582 issued June 16, 1981; Appleby, U.S. Pat. No. 4,184,930, issued Jan. 22, 1980; Fritts, U.S. Pat. No. 4,399,005, issued Aug. 16, 1983; McHenry, U.S. Pat. No. 3,355,325, issued Nov. 28, 1967; McHenry, U.S. Pat. No. 3,600,226, issued Aug. 17, 1971; O'Sullivan, U.S. Pat. No. 3,966,494, issued June 29, 1976; Stephenson, U.S. Pat. No. 3,986,893, issued Oct. 19, 1976; Seiger et al, U.S. Pat. No. 4,120,757, issued Oct. 17, 1978; Stark, U.S. Pat. No. 3,288,643, issued Nov. 29, 1966; Smith, U.S. Pat. No. 4,269,670, issued May 26, 1981.
Metal hydroxide electrodes generally comprise a plaque plus an electrochemically active material. A plaque is a porous, often sintered, physical support. The electrochemically active material is deposited within the pores of the plaque. Plaques are generally electrochemically inactive, yet are sufficiently conductive to pass the current used in the electrochemical process. Plaques are suitably metallic and generally chemically inert to the electrolyte used in the electrochemical process. The process by which the electrochemically active material is deposited in the plaque is referred to as impregnation.
Nickel hydroxide electrodes have been used in various types of batteries, particularly rechargeable energy storage batteries including nickel-cadmium, nickel-zinc and nickel-hydrogen couples. During the energy producing phase of the cycle, i.e., discharge of the battery, the nickel electrode acts as the positive oxidizing electrode. During charging phase, i.e., when energy is applied to the battery, the nickel hydroxide electrode acts as the negative or reducing electrode. The nickel electrode reaction may be described as: ##STR1## Nickel hydroxide electrodes generally comprise a porous, often sintered, nickel plaque as the current collecting substrate for the electrochemically active nickel hydroxide material. This porous plaque is then impregnated by precipitating finely divided nickel hydroxide in the pores of the plaque. Numerous impregnation processes are available, including electrodeposition, polarization and thermal decomposition. In electrodeposition, for example, the nickel plaque is cathodically polarized in nickel nitrate solution liberating hydrogen in the pores of the nickel plaque. The pH increases inside the plaque and nickel hydroxide precipitates. These reactions may be described as: EQU 2H.sub.2 O+2e.sup.-.fwdarw.H.sub.2 +2OH.sup.- EQU Ni.sup.2+ +2OH.sup.-.fwdarw. Ni(OH).sub.2
Often the nickel nitrate solution contains additives such as cobalt or cadmium ions, generally 5-10 percent of the nickel concentration, to increase life cycle time and increase utilization of active material
In the prior art, after impregnation, the electrodes undergo formation and characterization steps. These steps are generally considered essential for maximum cell performance and consist of electrolytic cycling in alkali metal hydroxide electrolyte. In the formation treatment, the electrodes are immersed in an alkaline solution, usually 25-35 percent by weight alkali metal hydroxide, typically sodium or potassium hydroxide, and are charged and discharged for 3 to 4 cycles. This formation step may take as long as 24-48 hours. It is conventionally performed to clean and remove impurities, such as nitrates, carbonates and loosely adhering particulates and to give the active material electrochemical "exercise" by an oxidation-reduction process. The characterization step also consists of charge-discharge cycling but at a much slower rate than the formation step; one cycle may take 24 hours. Characterization cycling also uses 25-35 percent alkali metal hydroxide with or without renewal from the formation step. The electrodes may be characterized for as many as twelve cycles.
For the purposes of this invention, the terms "formation" and "characterization" are intended to define electrolytic cycling in alkali meta hydroxide electrolyte with the purpose of improving the electrochemical properties of the battery electrodes.
These conventional techniques have several disadvantages. First, the entire production process is very time-consuming. Production of a nickel hydroxide electrode may take several days. Impregnation processes other than the electrochemical method may make production times even longer. Second, the alkaline solution used for the formation and characterization steps is generally discarded after each use because reuse of the alkaline electrolyte leads to low electrode characterization capacities. This constant renewal of electrolyte prolongs production and contributes to the high cost of making nickel hydroxide electrodes. Long production times and high costs constitute important obstacles to large scale commercial production of such electrodes. Thus, it is desirable to develop simpler, more rapid and less costly production methods for the electrodes.
Experimenters and battery manufacturers have found that impurities in the battery system play a major role in reducing charge retention and shortening shelf-life and cycle-life. It has been generally recognized that the presence of nitrate ions in nickel and cadmium electrodes can retard battery performance. Falk & Salkind, Alkaline Storage Batteries (Wiley 1969) p. 631-632. In a nickel-cadmium battery, nitrate ions are readily reduced to nitrite at the negative cadmium electrode. These nitrite ions readily diffuse to the positive nickel electrode where they are oxidized to nitrate. The overall reaction may be described as: EQU NO.sub.3 -+H.sub.2 O +2e.sup.- .fwdarw.NO.sub.2 -+2OH.sup.-
This nitrate-nitrite shuttle thus discharges both electrodes and shunts off some of the power of the battery. The rate of discharge will depend upon the nitrate concentration and factors determining the rate of diffusion of the ions, such as cell geometry, temperature and permeability of the separator.
Although the effect of nitrate has been studied primarily in the nickel-cadmium cells, a similar shuttle can exist in silver-zinc, silver-cadmium, nickel-iron and nickel-zinc systems also. The nitrate problem may be exacerbated in the nickel-cadmium system because the cadmium electrodes are also impregnated in a nitrate solution. A corrosive effect of nitrate on the nickel sinter of nickel hydroxide electrodes has also been reported. King et al., Proc. Ann. Power Sources Conf., vol. 16, p. 108 (1962). Nitrate is also reported to attack the silver of silver electrodes, gradually converting it into a colloid. Andre, communication presentation, 3rd Section of French Society of Electricians Meeting, Mar. 13, 1941.
Impregnation processes occur in substantially high amounts of nitrate and result in residual nitrate ion on the electrodes. The active material of the nickel electrode, for example, is nickel hydroxide and is a fine, gelatinous precipitate. Nitrate ions are readily entrapped in the precipitate. Washing the electrodes may remove some of the surface nitrates. Soaking the electrodes in an alkaline bath and allowing the nitrates to diffuse out of the electrodes may also be helpful. This process is often used as part of a formation treatment. See, Falk & Salkind, Alkaline Storage Batteries (Wiley 1969) p. 79. As described above, the formation treatment is conventionally used to remove impurities such as nitrate. As the formation process proceeds, the electrolyte acquires increasing amounts of nitrate. Decontamination of the electrolyte by precipitation of nitrate salts is not a practical solution because of the high solubility of nearly all nitrate salts.
Several other nitrate removal methods have been described. One method involves including both a reservoir tank and bath for the alkaline solution. The alkaline electrolyte is circulated between the bath and reservoir and is electrolyzed in the reservoir to convert the nitrate to ammonia. See, e.g., Herman et al, U.S. Pat. No. 3,671,320, issued June 20, 1972.
Another method involves a chemical reduction of nitrate to nitrite and nitrogen oxides using a compound containing an enediol group as the reducing agent. See, for example, Owen, U.S. Pat. No. 3,582,403, issued Jan. 1, 1971. Methods which subject the impregnated plaque to high temperatures to decompose the nitrates have also been described. See, e.g., Nervik, U.S. Pat. No. 3,663,296, issued May 16, 1972; Pensabene et al, U.S. Pat. No. 4,139,423, issued Feb. 13, 1979.
Another method entails an oxidation conditioning of the nickel hydroxide plaque in alkaline solution. The nickel hydroxide of the nickel electrode is oxidized to form the nickel oxyhydroxide while the nitrate is reduced to ammonia at the counter electrode. See, e.g., Maskalick, U.S. Pat. No. 4,337,124, issued Jan. 29, 1982. Because the reduction of nitrate occurs at the counter electrode, the rate of reduction must depend on the diffusion of nitrate out of the impregnated plaque. As noted above, nitrate diffusion may be controlled by many factors. Back diffusion of the ammonia to the nickel electrode may also occur resulting in the ammonia being oxidized to nitrate. This back diffusion may be minimized by controlling the current density so that the rate of ammonia produced is fast enough to keep the zinc around the counterelectrode saturated with ammonia, facilitating expulsion of ammonia from the solution before significant back diffusion occurs.
In a method for producing cadmium electrodes, the plaques may undergo an optional step which resembles polarization impregnation. After impregnation via electrodeposition, without washing and drying, the plaques are immediately placed in 15-30 percent potassium hydroxide, and cathodically polarized for 15-45 minutes. The residual nitrates are converted to ammonia and hydroxide. See, e.9., Pickett, U.S. Pat. No. 3,873,368, issued Mar. 25, 1975; see also, Falk & Salkind, Alkaline Storage Batteries (Wiley 1969) pp. 125-128 on polarization impregnation.
Despite recognition of the nitrate impurity problem, the foregoing methods require either an additional step in the electrode production process, require large volumes of reagents which often require frequent renewal, or rely on diffusion of the nitrate out of the electrode pores. The present invention addresses these problems in the known methods of nitrate removal and provides a method which can reduce production time and cost for metal hydroxide electrodes.